In this explainer, we will learn how to describe the components of the human genome and outline why scientists want to study the human genome.
Have you ever wondered what makes us, us? What determines if we will have brown or red hair, if we will have blue or green eyes, or even if we will have dimples when we smile?
The answer to this, in short, is our genome. Our genome is all of the genetic material within our cells. In humans, genetic material takes the form of DNA, with long molecules of DNA coiling and condensing into structures called chromosomes. We can see the relationship between DNA and chromosomes in our body cells in Figure 1.
The genome is all of the genetic material of an organism.
In the nucleus of a typical human body cell, also called a somatic cell, there are 23 pairs of chromosomes (making up 46 in total). Located on the chromosomes are hundreds, if not thousands, of genes. Genes are sections of DNA that code for a functional unit, typically a protein. Genes come in different versions called alleles.
A gene is a section of DNA that contains the information needed to produce a functional unit, for example, a protein. It is the functional unit of heredity.
An allele is an alternative version of a gene.
The 22 pairs of non-sex chromosomes in your body (somatic) cells are numbered according to their size. Non-sex chromosomes can be referred to using their chromosomal pair number. Figure 2 shows a karyotype, which is an image produced to help us visualize the chromosomes within a cell. You can see in Figure 2 that chromosome 1 is much larger than chromosome 22. The smaller the number used to name the chromosome pair, the larger the chromosome. For example, chromosome 1 (meaning the first pair of chromosomes) is the largest of all non-sex chromosome pairs. It is estimated to carry just over 2000 genes. However, the only exception to this general rule is chromosome 22. It was believed chromosome 22 was the smallest of the non-sex chromosomes, and that is why it was named 22, until it was discovered that, in fact, chromosome 21, which is estimated to carry between 200 and 300 genes, is a little bit smaller than chromosome 22, which is estimated to carry between 474 and 496 genes. The 23rd pair of chromosomes represents the sex chromosomes, whose combination determines the person’s biological sex: XX for female and XY for male.
A homologous chromosome pair is a pair of chromosomes that have similar lengths and have the same genes at the same location. For the homologous pairs of chromosomes in your body cells, one chromosome in each pair is inherited from your biological mother and the other is inherited from your biological father. This is because the sex cells that combine during fertilization (sperm and egg cell) contain only half the number of chromosomes that a typical body cell has. A gamete does not contain 23 pairs of chromosomes (so 46 in total), but rather one chromosome from each of the pairs. When these sex cells, or gametes, combine, they then have the correct number of chromosomes, 46. This process is briefly outlined in Figure 3.
Inheriting one set of chromosomes from your biological mother and one set from your biological father means that you will likely end up looking like a mix of both of them.
Example 1: Identifying the Key Characteristics of Chromosomes in Humans
Which of the following statements about human chromosomes is true?
- Each chromosome in a somatic (body) cell contains the entire human genome.
- All chromosomes are the same size and contain the same number of genes.
- A somatic (body) cell contains half the number of chromosomes that a gamete (sex cell) contains.
- A single chromosome can carry hundreds to thousands of genes.
All the genetic material within an organism is called its genome. Our body contains huge amounts of genetic material in the form of DNA. Long molecules of DNA coil and condense to form structures called chromosomes.
In a typical human body cell, there are 46 chromosomes arranged into 23 homologous pairs. Homologous pairs of chromosomes are typically very similar in size and carry a very similar number of genes. The first 22 pairs of chromosomes are numbered and arranged in terms of their size. The largest of them, chromosome 1, is estimated to carry just over 2 000 genes. The smallest non-sex chromosome, chromosome 21, carries only just under 300 genes. The 23rd pair of chromosomes represents the sex chromosomes, which are responsible for determining the biological sex of the person. Typically, the combination XX produces a female, whereas the combination XY produces a male.
Alongside body cells, such as skin cells, muscle cells, and the cells of our other organs, the human body produces sex cells. These sex cells, also called gametes, do not contain 46 chromosomes. Instead, they contain one chromosome from each of the pairs, so just 23 in total. This is so when the sex cells combine during fertilization, the resulting embryo has the correct number of chromosomes in each of their cells.
Let’s use this information to identify which of the above statements is correct.
We know that the human genome is all of the genetic material within the human body, not just that contained in a single chromosome, so option A is incorrect. By looking at chromosomes 1 and 22, we know that chromosomes vary significantly in terms of their size and the number of genes they carry, so option B is also incorrect. A body cell contains 46 chromosomes, whereas a sex cell (or a gamete) has half this number, only containing one chromosome from each of the 23 pairs. So option C is also incorrect.
We know that chromosomes can carry many, many genes. We mentioned earlier that chromosome 1 is thought to carry around 2 000 genes and that chromosome 21 carries just under 300 genes.
Therefore, our correct answer must be option D. A single chromosome can carry hundreds to thousands of genes.
Let’s have a look at some of the scientific research and discoveries that have furthered our understanding of the human genome.
In the 1950s, multiple groups of scientists started working to propose a model explaining the structure of DNA molecules. At King’s College London, Rosalind Franklin was using a technique called X-ray crystallography to study the structure of DNA within the nuclei of cells. Franklin and one of her students managed to produce an X-ray diffraction image, called Photo 51, which provided crucial evidence that DNA forms a double helix.
Without her consent or knowledge, Franklin’s image of DNA was shared with Francis Crick and James Watson at the University of Cambridge. They used this image to finalize their proposed model of DNA taking the form of a double helix in 1953.
One of the reasons why determining the shape of DNA was so important is the discoveries and subsequent research that this led to. Before the 3D shape of DNA was confirmed, it was already known that each molecule of DNA is made up of repeating units called nucleotides. These nucleotides contain a sugar-phosphate backbone, with bonds joining adjacent nucleotides, and the information is contained within their nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases, along with the basic structure of a double-stranded DNA molecule, are shown in Figure 4. The particular sequence of these bases and how they are read to create proteins in organisms are known as the genetic code. After his contribution to the discovery of the DNA structure, Francis Crick and other scientists provided evidence that a sequence of bases is read in groups of three during the process of creating proteins.
Once scientists understood the shape of DNA and how we can read DNA sequences to determine what amino acids, and therefore proteins, will be produced, this allowed them to start sequencing genomes. Genome sequencing is the process by which the order of DNA bases in a genome is determined.
Key Term: Genome Sequencing
Genome sequencing is the process of determining the sequence of DNA bases in a genome.
Figure 5 provides a timeline of some of the most important discoveries in our journey to understand the human genome, including major sequencing events.
Example 2: Recalling Significant Scientific Events in Understanding the Human Genome
A sequence of events has increased our understanding of the human genome. Which of the following events came first?
- The Human Genome Project sequenced the entire human genome.
- The double helix model of DNA was proposed and accepted.
- Chromosome 22 was fully sequenced.
- The first single-celled organism had its genome sequenced.
The genome of a human is all of the genetic material of that individual and the information that material contains. We have huge amounts of genetic material within our bodies in the form of DNA. Long molecules of DNA coil and condense to form structures called chromosomes. On these chromosomes are hundreds to thousands of genes. A gene is a section of DNA that codes for a particular functional unit, for instance, a protein.
Throughout history, there have been a series of crucial biological discoveries and research projects that have helped advance our understanding of the human genome. One of the largest advances of the 20th century was the proposal of the double helix model of DNA, which provided valuable insights into the shape and structure of DNA within our cells. This model was proposed by Watson and Crick, using the work of Franklin and Wilkins, in 1953.
Since then, scientists have built on this discovery by creating genetic maps of organisms and sequencing the DNA within chromosomes and whole single-celled organisms. Eventually, in 2003, the project of sequencing the entire human genome was officially completed.
Therefore, of the options given, we can conclude that the first discovery is that the double helix model of DNA was proposed and accepted.
One of the largest and most important biological research efforts in human history is the Human Genome Project.
The Human Genome Project was an international collaboration between many groups of scientists aimed at determining the sequence of all the DNA bases in the human genome and accurately map all of the human genes present. This was a huge task, as the average human genome contains approximately 3 billion base pairs. This is a huge number; if you recited the human genome at a rate of one base per second, for 24 hours a day, it would take you around 100 years to say it out loud.
Sequencing the human genome has incredibly valuable applications in the field of medicine. By identifying and mapping genes, scientists can locate genes that cause disease or make a person more susceptible to a particular disease. It is also possible to identify genes, or their versions (alleles), that react negatively to a particular medicine or reduce its effectiveness. This means that medicine can potentially be targeted for different individuals.
Fact: The Human Genome Project
The Human Genome Project successfully determined the order of 3 billion base pairs in the human genome. It was launched in 1990 and was completed in 2003, and it involved the collaboration of many scientists from a wide range of disciplines from all around the world.
Over time, our knowledge of DNA, genes, chromosomes, and genomes has increased. Scientists have discovered more and more genes in our genome, and we now believe that humans have around 21 000 protein-coding genes located across 23 pairs of chromosomes.
Example 3: Understanding and Explaining the Benefits of Studying the Human Genome
Epidemiologists are scientists who study the causes, distribution, and consequences of diseases in specified populations. Which of the following best explains why an epidemiologist might want to study the human genome?
- To determine which genes in humans can cause diseases or make a person at risk of catching a disease
- To determine that DNA is the genetic material in humans
- To create new variants of genetic diseases
- To genetically modify the human genome to become more susceptible to the transmission of diseases
The human genome refers to all of the genetic material and the information that material contains within a human. By studying the human genome, we can better understand our own genetics. It also has many useful applications in the field of medicine. By knowing the entire collection of genes in the human genome, we can determine genes that cause disease or make a person more susceptible to developing a disease. We can also investigate genes that may inhibit or restrict the effectiveness of certain drugs and use this knowledge to produce medicines tailored to specific people and their needs.
Looking back at our options, we can eliminate some choices. Creating new variants of genetic diseases is incredibly dangerous and not something that an epidemiologist, or in fact any scientist, would want to do. Genetically modifying the human genome to become more susceptible to disease is also dangerous and detrimental to human health, so we can also eliminate this choice. We already know that the DNA is the genetic material of humans; this was confirmed by a series of experiments in the 20th century but not by epidemiologists studying the human genome.
Therefore, we can conclude that epidemiologists may want to study the human genome to determine what genes in humans can cause disease or make a person at risk of catching a disease.
The discoveries of the human genome also have valuable applications in other areas of science. For instance, sequencing the genomes of other organisms has allowed us to compare the genomes of different species and provided valuable evidence for how organisms have evolved over time.
Let’s recap some of the key points that we have covered in this explainer.
- The human genome is all of the genetic material and the information it contains within a human.
- DNA takes the form of a double helix within the nuclei of human cells.
- DNA coils and condenses to form chromosomes, which carry hundreds to thousands of genes.
- Genes are sections of DNA that code for functional units, for example, proteins, and they are inherited from the parents to the offspring.
- Important discoveries and research projects in advancing our understanding of the human genome include the discovery of the structure of DNA, the identification of genes, and the Human Genome Project.
- The main goals of the Human Genome Project were to identify all the genes within the human genome (including those that cause disease) and study the evolution of living organisms.